1. Field of the Invention
The present invention relates to an injection mold having a mechanism for shearing a film gate within the die. More particularly, the present invention is concerned with a configuration of the gate which is capable of optimizing the flow rate and the pressure of the resin supplied into the cavity.
2. Description of the Related Art
A die for molding an optical disk will be described as an example of a die having a mechanism for shearing a film gate within an injection mold.
Referring to FIG. 2, a known injection mold has a gate which is defined by the end surface a of a gate cutter 1 and the end surface b of a sprue bush 2. In this mold the gap between the end surfaces a and b and the length l of the lands presented by the above-mentioned end surfaces are determined to stabilize the flow of the flowing resin and optimize the application of pressure.
The cutting of the gate is effected by a forward movement of the gate cutter 1 as indicated by an arrow in FIG. 3, while allowing the sprue bush 2 to be retracted. This in turn requires that the touch pressure of the nozzle 3 of the injection molding apparatus is reduced or the nozzle 3 is forcibly retracted. Anyway, a delicate and intricate control is necessary.
If the shearing has to be conducted by the forward movement of the cutter 1 alone, it is necessary that the sprue bush 2 is set at the retracted position as shown in FIG. 4. In this case, the gate is defined by the clearance between the inner peripheral corner c of the dies 4 and the outer peripheral end d of the gate cutter 1. Namely, the land l shown in FIG. 2 is substantially zero so that the effect for stabilizing the flow of the resin and the application of the pressure to the cavity 5 depends solely on the thickness of the gate.
With this type of injection mold, therefore, it has been extremely difficult to produce a product having an essential characteristic which is worsened in accordance with a rise in the molding pressure, e.g., birefringence in case of an optical disk, since the production of such product requires a highly delicate control of the molding pressure.